1. Field of the Invention
The present invention relates to a motor-driven injection molding machine having an injection device driven by a servo motor. More particularly, the present invention relates to a method for adjusting the zero point for a load cell for use in pressure detection which is provided to form a control system for the servo motor.
2. Description of the Related Art
Motor-driven injection molding machines have increasingly been used in recent years in which a servo motor is used as a substitute for a hydraulic actuator. Operation of such injection device using servo motors can be summarized as follows.
A screw is rotated with a servo motor for screw rotation in a plasticization/metering process. The screw is located within a heating cylinder. A resin is fed from a hopper to a rear portion of the screw in the heating cylinder. Rotation of the screw melts and advances the resin and thus a certain metered amount of resin is fed to a nose portion of the heating cylinder. During this time, the screw is driven backward due to a back pressure of the molten resin trapped in the nose portion of the cylinder.
An injection shaft is directly connected to a rear end portion of the screw. The injection shaft is rotatably supported by a pressure plate through a bearing. The injection shaft is driven in the axial direction with a servo motor for injection that is supported on the pressure plate. The pressure plate moves forward and backward along guide bars in response to the operation of the servo motor for injection through a ball screw. The above-mentioned back pressure of the molten resin is detected by using a load cell and is controlled with a control loop.
In a filling process, the pressure plate is advanced by means of driving the servo motor for injection. The nose portion of the screw serves as a piston to fill a mold with the molten resin.
The molten resin fills the space within a mold cavity at the end of the filling process. At this point, the control mode for the advancing motion of the screw is switched from a velocity control mode to a pressure control mode. This switching is referred to as V(velocity)-P(pressure) switching and affects the quality of a resultant molded product.
Following the V-P switching, the resin within the mold cavity is allowed to cool under a predetermined pressure. This process is referred to as a dwelling process. In this dwelling process, the pressure of the resin is controlled in a feedback loop as in the above-mentioned back pressure control.
Then, the injection device returns to the plasticization/metering process set forth above after the completion of the dwelling process. On the other hand, a clamping device performs an eject operation for ejecting a solid product out of the mold in parallel with the plasticization/metering process. The ejection operation involves in opening the mold to remove the solid product from the mold by means of an ejector mechanism, and then closing the mold for the resin filling noted previously.
For the injection molding machines having the load cell, it should be noted that a zero point for the load cell is required to be adjusted upon the start-up of the injection molding machine. The adjustment of the zero point is done, for example, as follows. The screw is advanced once and is retracted once to calculate an average value of detected values obtained by using the load cell during the back and forth movement of the screw. The average value is set as the zero point for the load cell.
However, the method as described above which relies on only a single reciprocation of the screw for the zero point adjustment does not necessarily provide an accurate zero point. One possible reason for this is that a solidified resin may accidentally exist between the screw and the heating cylinder during the single reciprocation movement of the screw, inhibiting the complete movement of the screw. Another reason may be due to foreign material accidentally entrapped in a movable unit or units connected mechanically with the screw. In the cases described above, it is hard to detect the back pressure with the load cell under the influence of the solidified resin or the foreign material. It can easily be expected that such a failure of setting the accurate zero point for any reasons including those described above badly affects the quality of a resultant molded product obtained in a subsequent molding process.